1. Field of the Invention
This invention relates to an electromagnetic switch, and more particularly, to an improvement in a lead-out arrangement for leading out one end of a coil wound around a bobbin of the electromagnetic switch.
2. Description of the Prior Art
In the past, various electromagnetic switches have been well known, a typical example of which is described in Japanese Utility Model Laid-Open No. 56-37318 (1981). The general arrangement of such a conventional electromagnetic switch is illustrated in FIG. 5. The electromagnetic switch illustrated includes a cylindrical outer casing 1 closed at one end by an end wall 1a and open at the other end. The end wall 1a of the outer casing 1 has a central opening through which a plunger 2 is inserted into the casing 1 for axial sliding movement relative thereto. Disposed at the open end of the outer casing 1 is a stationary iron core 3 in a face-to-face relation with the plunger 2, the stationary iron core 3 having an end wall 3a which is fitted into and fixedly secured to the inner periphery of the casing open end so as to constitute a switch housing for receiving a coil bobbin 4 therein. The coil bobbin 4 is formed of synthetic resin, and has a cylindrical member 4a and a pair of annular flanges 4b integrally formed with the cylindrical member 4a at its opposite ends in such a manner as to oppose the end wall 1a of the outer casing 1 and the end wall 3a of the iron core 3, respectively.
An exciting coil 5 is wound around the bobbin 4 and adapted to magnetize, upon energization thereof, the iron core 3 received therein so that the iron core 3 thus magnetized acts to magnetically attract the plunger 2. A return spring 6 is disposed under compression between the iron core 3 and the plunger 2 for biasing the plunger 2 in a direction away from the iron core 3. Inserted into a through-hole formed through the iron core 3 at its central portion is a rod 8 which has a movable contact 7 at its tip end. A contact cap 9 formed of an electrically insulating material is fixedly attached by caulking to the open end of the outer casing 1 together with the end wall 3a of the iron core 3.
The conductive wire of the exciting coil 5 wound around the bobbin 4 has one end extending to the outside from a lead-out portion 10 formed over one of the bobbin flanges 4b and the iron core end wall 3a through an eyelet 12 which serves to secure a terminal 11 to the contact cap 9. As clearly shown in FIGS. 6 and 7, the lead-out portion 10 is comprised of a pair of projected guide walls 4c integrally formed with and extending axially outwardly from the bobbin flange 4b along a notch 3b formed in the iron core end wall 3a. The projected guide walls 4c are connected with the bobbin flange 4b through connecting corner portions, each of which is tapered or chamferred at 4d as pictured in FIGS. 6 through 8 so as to prevent injury or damage to the covering film or insulating layer on the surface of the lead-out end of the conductive wire of the exicting coil 5.
With the lead-out portion 10 of the conventional electromagnetic switch as constructed above, however, the corners connecting beteen the projected guide walls 4c and the bobbin flange 4b are merely tapered or chamferred at 4d over a relatively small area so that the lead-out end 5a of the coil wire is forced to curve or bend at the connecting corners with a relatively small radius of curvature. As a result, there is the great possibility of the covering film or insulating layer on the coil wire lead-out end being injured or damaged, and hence the conventional lead-out portion 10 is not satisfactory.
On the contrary, it has been considered to increase the thickness of the bobbin flange 4b in order to taper or chamfer the connecting corners of the lead-out portion 10 over a relatively larger area so as to permit the lead-out end 5a of the coil wire to curve at a relatively large radius of curvature. In this case, however, there is the problem that the entire length of the bobbin 4 would become greater, thus increasing the size of the electromagnetic switch itself. On the other hand, it has further been considered to reduce the thickness of the iron core end wall 3a by the increased amount of thickness of the bobbin flange 4b. In this case, as the plunger 2 is magnetically drawn toward the iron core 3 under the action of magnetic attraction force which is generated by a magnetic circuit passing through the outer casing 1, the end wall 3a and the cylindrical body of the iron core 3, and the plunger 2 when the exciting coil 5 is energized, as shown by a dotted line 13 in FIG. 5, if the cross sectional area of the magnetic circuit 13 is reduced at the thinner iron core end wall 3a, the magnetic resistance will increase at that location, thus posing a problem in that the size of the exciting coil 5 would have to be enlarged so as to produce greater magnetomotive force to compensate for the increased magnetic resistance.
Thus, in the conventional lead-out arrangement as described above, it is difficult to lead out the end of the conductive wire of the exciting coil 5 by permitting it to curve at a relatively large radius of curvature, and to cope with this, the tapered or chamferred area at the connecting corners between the projected guide walls 4c and the bobbin flange 4b is enlarged as much as possible. With this measure, however, the possibility of injury or damage to the covering film or insulating layer on the coil wire lead-out end is not practically reduced, and such a measure is still not satisfactory.